This invention relates to clarifiers and thickeners of the deep bed type. This invention also relates to an associated method for separating liquid and solid particles in a suspension of slurry in thickness and clarifiers.
Deep bed thickeners and clarifiers (or so-called thickener/clarifier devices) are shown and described, for example, in U.S. Pat. Nos. 5,433,862, 5,549,827 and 5,800,715, the disclosures of which are hereby incorporated by reference herein.
The device described in U.S. Pat. No. 5,433,862 comprises a vessel receiving a slurry of liquid and solid particles suspended on the liquid. The vessel defines a hindered settling zone in which the liquid and solids of the slurry tend to separate with the solids settling down into a compaction zone below the hindered settling zone. In the compaction zone, the solids concentration tends to increase, while the liquid phase rises to a clarification zone above the hindered settling zone. An overflow launder adjacent the upper end of the vessel receives the overflow phase of the slurry separated in the vessel. An underflow discharge port toward the bottom of the vessel receives the thickened or underflow phase of the slurry separated in the vessel. One or more so-called separating members are provided in the vessel extending upwardly from a lower end in the hindered settling or compaction zones to an upper end in a clarification zone. Preferably, this member includes cones or other downwardly inclined surfaces that facilitate both the settling of solids down to the compaction zone and the upward flow of free liquid from the hindered settling and/or compaction zone to the clarification zone.
The deep bed thickener or clarifier device described in U.S. Pat. No. 5,800,715 is similar to that disclosed in U.S. Pat. No. 5,433,862, but further has a fluid flow barrier between the overflow clarification zone and the other zones of the vessel, and one or more clarifying conduits extending through the barrier. Each such conduit carries a suspended floc bed of particulate solids that filters and further clarifies the liquid flowing up from the hindered settling zone to the clarification zone. This further filtering removes solid particles that would otherwise be carried along the xe2x80x9cclarifiedxe2x80x9d liquid to the overflow launder. These clarification conduits thus serve a significantly different purpose than do the separating members and function independently thereof.
Deep bed thickener/clarifiers offer certain design and operational advantages over the conventional thickeners and clarifiers while providing clarity of overflow and solids concentration of underflow comparable to that of conventional thickener/clarifiers. Such thickeners and clarifiers are typically of low-profile design (i.e., shorter in height but of much greater diameter than deep bed thickeners), but like deep bed thickeners have generally central feedwells and underflow underflow discharge ports.
Conventional low profile thickeners rely on the radial expanse of the horizontal settling zones to effect settling of the slurry throughout the thickener. To achieve this settling action, the influent feed is typically dispersed radially outwardly from the central feedwell and rotating rake arms, having generally plow shaped blades, gather and laterally direct the settled solids along the thickener/clarifier bottom to the central discharge opening in the bottom of the thickener/clarifier tank. These rake blades are located closely adjacent the tank bottom and are designed to move the solids across the tank bottom with minimal disruption in the downward (or settling) flow pattern of the slurry though the hindered settling and compaction zones. Consistent with this purpose, the rakes are rotated at a very slow speed for example, less than 1 revolution per minute. U.S. Pat. Nos. 4,082,671, 4,217,916 and 4,271,027, for example, describe and illsutrate rake assemblies that perform this function of moving settled solids across the tank bottom of conventional low-profile thickeners for discharge at a central underflow outlet. Indeed, some of the rake assemblies are intended to scrape the settled solids from the tank bottom, see U.S. Pat. No. 4,247,400.
In contrast, xe2x80x9cdeep bedxe2x80x9d thickener/clarifiers rely on the height of the hindered settling to cause separation of the slurry into its clarified liquid and settled solids phases. Deep bed thickeners/clarifiers further have a deep, high angle conical bottom for directing the settled solids toward the discharge opening. Thus there is no need for rotating rakes to move the solids within the thickener/clarifier to effect removal of the solids. Indeed, rakes would serve little purpose in moving the solid, but rather would add expense and operating complexity.
Basically, deep bed thickeners rely on a conical tank bottom having a sidewall extending an angle from horizontal generally greater than the angle of repose of the settled solids to direct the solids to move by gravity alone along the tank bottom to the discharge opening. This eliminates the need for a rotating rake and thereby simplifies the operation and construction of the thickener/clarifier as well as reduces the cost of the thickener/clarifier compared to conventional thickeners/clarifiers. This cost reduction takes the form of the elimination of a capital expenditure for the rake assemblies, as well as the elimination of the associated torque driveheads and motors (often in excess of 1,000,000 foot pounds torque) and high strength bridges spanning the tank top to support the drive.
In contrast, conventional low profile thickener/clarifiers have tank bottoms that are flat or inclined at angles generally below the angle of repose of the settled solids and thus require rake assemblies and the associated drive and bridge (or column) structures to remove the settled solids from the thickener/clarifier.
The present invention enhances liquid-solids separation in a deep bed thickener/clarifier and thus produces an underflow product or sludge which is thickened relative to that of existing deep bed thickeners/clarifiers.
More particularly, the deep bed thickener/clarifier of this invention provides for the release and removal of free or interstitial liquid (typically water) trapped in the settled solids in the hindered settling zone and/or the compaction zone. This release of liquid is effected without increasing the size of the thickener/clarifier, disturbing the settling processes or the naturally occurring movement of the solids along the conical tank bottom.
The deep bed thickener/clarifier of the present invention comprises a vessel with an overflow launder and an underflow discharge port. The vessel has an inlet for receiving a slurry of a liquid and solid particles suspended in the liquid. In an upper portion of the vessel, a free settling zone has a low concentration of solid slurry particles, while in a lower portion of the vessel, a compaction zone has a high concentration of solid slurry particles. The overflow launder is attached to the vessel adjacent an upper end thereof for discharge of an overflow phase of the slurry that has been separated in the vessel. The underflow discharge port is adjacent to a bottom thereof for discharge of a thickened, underlow phase of the slurry that has been separated in the vessel.
An upper portion of the vessel holds the free settling zone and a lower portion of the vessel holds the compaction zone. A hindered settling zone extends between the free settling and compaction zones, with the hindered settling zone and compaction zone constituting lower settling zones. A deliquifying member is positioned in the vessel extending up from adjacent the lower portion of the vessel to adjacent the upper portion of the vessel for flow of liquid from at least one of the lower zones to the free settling zone. The deliquifying member further serves to isolate this flow of liquid from the flow of slurry down in the lower settling zones. A rake assembly is mounted for rotation in the lower portion of the vessel, with the rake assembly forming channels in the slurry held in the lower portion of the vessel for releasing liquid in at least one of said lower settling zones to flow to the deliquifying member and on to the free settling zone.
The method of this invention for operating a deep bed thickener/clarifier comprises, in accordance with the present invention feeding, into a vessel, a slurry of a liquid and solid particles suspended in the liquid and separating the slurry by gravity into different zones having respective degrees of liquid-solids separation including, at an upper portion of the vessel, a free settling zone having a low concentration of solid slurry particles and further including, at a lower portion of the vessel, a compaction zone having a high concentration of solid slurry particles. A hindered settling zone is positioned between these zones. The method also comprises discharging, into an overflow launder attached to the vessel proximately to an upper end thereof, an overflow phase of the slurry that has been separated in the clarification zone and discharging, via an underflow discharge port fixed to the vessel at least proximately to a bottom thereof, a thickened, underflow phase of the slurry that has been separated in the vessel. Liquid is directed from at least one of the lower settling zones to the free settling zone via a flow path which is isolated from the flow of slurry down in the vessel in the lower settling zones. In accordance with his invention, upwardly, extending channels are formed in the slurry held in the lower portion of the vessel to release liquid in at least one of the lower settling zones to flow to the isolated flow path and then to the free settling zone.
Other objects of the invention will be in part apparent and in part described and shown in the following description of the drawings and preferred embodiments.